1. Field of the Invention
The present invention relates to a multilayer resist and a method of forming a resist pattern using the multilayer resist, and in particular, to a multilayer resist which enables to utilize excellent duplication capability of X-ray lithography, and to a method of forming a highly accurate resist pattern with improved positioning accuracy and alignment accuracy.
2. Description of the Prior Art
Conventionally, when a fine pattern is to be duplicated or transferred on a photoresist layer by a photolithography technique, the exposure using the G line or I line is carried out to obtain high resolution. Furthermore, recently, for example, excimer laser lithography, phase shift technique, and the like have been developed for a photo-process for a semiconductor device which is required to be finer in structure. However, in these excimer laser lithography, phase shift technique, and the like, if the film thickness of the photoresist is thin, even a considerably fine pattern can be formed with high accuracy. In this case, however, a problem is involved in that the resist having such a thin film thickness cannot withstand various treatments or works in an LSI manufacturing process. Moreover, there is another problem in that it is difficult to obtain a practical pattern of 0.25 .mu.m or less in the above-mentioned techniques.
Accordingly, as a method for obtaining a practical pattern of 0.25 .mu.m or less, a proximity technique using X rays (wavelength=4 to 20 .ANG.) is proposed. In this technique, X rays are radiated onto a photoresist layer through a transmission type equal-magnification mask having a desired pattern formed thereon, so that the pattern on the transmission type equal-magnification mask is duplicated or transferred to the photoresist layer. This technique is confirmed to be an excellent duplication technique in which very high resolution is obtained, and even a photoresist layer having a large film thickness can be formed with a very correct shape. However, since the above-mentioned X-ray proximity lithography employs equal-magnification projection, there is a problem in that the accuracy required for the mask is very severe, and the positioning accuracy and the alignment accuracy are insufficient. Thus, a manufacturing technique for a mask with high accuracy has not yet been established.
Accordingly, in order to solve such problems, an X-ray reduction lithography is proposed in which a reflection type mask is used, and the mask is irradiated by X rays, and a photoresist layer is irradiated by the reflected light. In this method, a pattern of 0.05 .mu.m can be duplicated with high accuracy, and since reduced-magnification projection is used, the mask can be manufactured easily. However, due to the limitation to a multilayer film which can be used as an X-ray reflecting mirror, the wavelength of X rays usable at present is 100 .ANG. or larger. The X rays having this wavelength of 100 .ANG. or larger are absorbed in a photoresist layer to a great extent. For this reason, for example, when X rays having a wavelength of 100 to 200 .ANG. are used, only a pattern having a film thickness of about 0.3 .mu.m is obtained. Thus, there is a problem in that it is impossible to perform patterning of a photoresist layer having a thick film thickness, and it is impossible to obtain a photoresist pattern having a required thickness.
In the X-ray reduction lithography using the X rays mentioned above, another patterning method is proposed. In this method, a photoresist layer containing Si is formed on a thick duplication photoresist layer which is used for duplication, and only the photoresist layer containing Si is exposed to carry out patterning. The photoresist layer containing Si after patterning is used as a mask, and the duplication photoresist layer is subjected to anisotropic etching (O.sub.2 -RIE) using oxygen thereby to perform patterning on the duplication photoresist layer. Here, the above-mentioned photoresist layer containing Si is formed to have a film thickness equal to or smaller than a depth of intrusion of the X rays used in the reduced-magnification projection exposure. In this method, since the duplication photoresist layer is patterned by the O.sub.2 -RIE, even when the X rays having a wavelength of 100 to 200 .ANG. is used, a photoresist pattern having a required thickness can be obtained.
However, in the recuded-magnification projection exposure method utilizing the O.sub.2 -RIE, the problem is involved in that the damage of a substrate due to the O.sub.2 -RIE is large, and the reliability of a semiconductor device is degraded. Furthermore, since the O.sub.2 -RIE is carried out within a vacuum apparatus, the throughput is poor.
A prior art method of suppressing the occurrence of the damage of the substrate is disclosed in Japanese Patent Laid-Open Publication Hei No. 1-191130. In this prior art method, in a process of forming on a matter to be treated a novolak positive resist layer constituting a first resist layer and a novolak positive resist layer constituting a second resist layer respectively having different wavelengths to which the resist layers are photosensitive, as the second resist layer, a novelak resist containing a photosensitive agent composed of 1,2-naphthoquinone diazide-5-sulphonic acid, and a light absorbing agent having a light absorbing property to a short wavelength of 420 nm is used. As a result, an aimed thicknes of the photoresist pattern can be formed without using the O.sub.2 -RIE process. However, it is difficult to obtain a practical pattern of 0.25 .mu.m or less in accordance with the prior art method disclosed in the Japanese Patent Laid-Open Publication Hei No. 1-191130, because of applying a mercury emission line for lithography